Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a scavenging portion and blowdown portion of exhaust gases. The timing of the first set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
However, the inventors herein have recognized further issues as a result of operation with such systems. As one example, at a part throttle condition (where an intake throttle is at least partially closed), flow in the EGR passage may be reversed and intake air may be introduced into engine cylinders via the EGR passage. This may cause decreased mixing and decreased cylinder balance. The inventors have further realized that the scavenge exhaust valves may be disabled to reduce the reverse flow through the system. However, this may require cam control systems that may increase engine costs. Additionally, engine operation with the scavenge exhaust valves deactivated may reduce fuel economy and increase engine emissions, as well as degrading engine warm-up capabilities during a cold start.
In one example, the issues described above may be addressed by a method, comprising: while both a first exhaust valve and second exhaust valve of a cylinder are open, routing intake air through a flow passage coupled between an intake passage and a first exhaust manifold coupled to the first exhaust valve; and further routing the intake air through the first exhaust valve, into the cylinder, and out of the second exhaust valve to a second exhaust manifold coupled to an exhaust passage including a turbine. As one example, the flow passage may be coupled to the intake passage downstream of a compressor and upstream of an intake throttle. As a result of the positioning of this passage, a pressure of the first exhaust manifold may increase above atmospheric pressure and thus above a pressure in the exhaust passage. This allows the intake air to be routed through the first exhaust valve, into the cylinder, and out the second exhaust valve. Routing the intake air through the cylinder in this way allows residual exhaust gas to be pushed out of the cylinder and into the exhaust passage prior to the closing of the second exhaust valve. As a result, engine efficiency and fuel economy may be increased, even at part throttle conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.